1. Field of the Invention
This invention relates to a conductor composition which is adapted to be applied for multilayered ceramic bodies typically represented by, for example, a multilayered ceramic substrate or a multilayered ceramic capacitor effective for rendering an electronic device small in size and light in weight, and a method of manufacturing a multilayered ceramic body using the composition.
2. Description of the Prior Art
Multilayered ceramic bodies and hybrid ICs have used for their conductor materials a noble metals such as Au, Pt, Pd and the like on a base metal such as W, Mo, Ni, Cu and the like.
The conductor pattern of the hybrid IC is formed in such a manner that an organic binder and a solvent are added to the above a metal to make a paste, the paste thus prepared is screen-printed on an insulation substrate made of, for example, alumina, and fired in a belt type furnace. In the case of the multilayered ceramic body, pasted ceramic or glass power is used as an insulating or dielectric material in addition to the conductor paste to make a multilayer structure through the screen printing process.
Also, another method is available which obtains a multilayered body by applying the above mentioned conductor paste on a green tape consisting of the above mentioned insulating powder and an organic binder in a laminating manner.
As metallic materials to be used in making multilayered ceramic bodies, Au, Ag and Pd can be advantageously fired in air, but are disadvantageously expensive because they are noble. On the contrary, W, Mo, Ni and Cu are all inexpensive because of the base metal, but it is necessary to subject them to a firing process under either a reduction atmosphere or under inert atmospheric conditions.
For the multilayered ceramic substrate, W and Mo have been generally well used. The firing process thereof, however, is carried out at a temperature as high as 1600.degree. C. under a reducing atmosphere.
Thus, with the development on a substrate material which is low in cost, high in production capacity and possible to be fired at a low temperature, Cu is now being watched with keen interest as a conductor material capable of providing a low conductive resistance and higher reliability. U.S. Pat. Nos. 3,977,887 and 4,301,324 disclose developmental examples of low temperature firing substrates.
In addition, for the multilayered ceramic capacitor, Pd has been used as the conductor material, but it is also disadvantageously expensive, so that such a method has been proposed that the dielectric body is fired at a low temperature and Ag, an inexpensive metal, is used substitutionally with a part of the Pd, or Ni is used as the conductor material. (See, for example, U.S. Pat. Nos. 4,115,493 and 4,451,869.)
However, Ni or Cu, which is a base metal, has such disadvantages that the firing cannot be carried out in air and that the nitrogen gas atmosphere during firing must be carefully controlled since the properties, such as adhesion to the material to be metalized, sheet resistance, solderability, and so on, and the dissolution of the organic binder contained in the paste must be considered at the same time. Namely, with the copper electrode, the low oxygen partial pressure of the nitrogen gas atmosphere cannot cause the dissolution of the organic binder to take place but allows it to remain in a carbonic form, which badly effects metallization. Inversely, if the oxygen partial pressure is high, the copper electrode will be oxidized.
Thus, it is required to control the oxygen partial pressure sophisticated manner, which makes it unsuitable to meet mass production requirements.
Thus, considering the fact that the use of a base metal such as Cu and Ni in the oxide form makes it possible to achieve complete removal of the organic binder in air, we have developed a new method effectively achieving both the binder removal and the copper metallization. In this method, a multilayered body is formed by using a conductor paste containing either CuO or NiO as the main component, the binder removal is carried out through heat treatment in air, and then the oxide is reduced to metal in a reduction atmosphere containing a hydrogen gas, followed by a final firing process under a nitrogen gas atmosphere. The method is disclosed in U.S. Pat. Nos. 4,695,403 and 4,714,570 and the U.S. Pat. application Ser. No. 018,579.
The method we developed offers many advantages in that the binder removal can be achieved completely, the atmosphere during firing can be easily controlled and further, the paste uses a stable oxide as the starting material. We must, however, indicate here that there exists a point to be improved. That is, when CuO is used as the original material, since the reduction process is carried out, as an intermediate step of the manufacturing method can produce a pure copper, reactivity with a material to be metalized reduces, so that an adequate adhesion strength is not obtained.
Therefore, we have adopted a method in which Bi.sub.2 O.sub.3, CdO, MnO.sub.2 or the like is used as the additive to the CuO paste in order to increase the adhesion strength. However, adequate adhesion strength is sometimes unobtainable when the firing temperature body is low, or when firing is required to be achieved in a short period of time.